For this video, we examine the zones in which calls are placed. From the interpretation I work from when I configure "this" particular environment, calls to the "Local Zone" are interpreted as being an dialed extensions that are locally configured "on the VCS" (i.e., ext. 4001 and 4002, for example). Extensions that are "not" locally configured on the VCS are looked as being "External," meaning configured on a "neighboring" device via a SIP trunk (more on that in the video).
I have to say, finally getting a basic understanding of video configuration will give you a sense of accomplishment for those of us transitioning over from "voice" to "voice and video" (UC Collaboration). The first time you see that video come across the screen, you'll realize you've just up'd your market value.